The present invention relates to an improved flocculation process for the concentration of suspensions. In particular flocculated solids can be settled to form a bed in which higher solids and/or reduced yield stress can be achieved.
It is known to concentrate suspensions of solids in aqueous liquids by use of flocculants resulting in flocculation of the solids which facilitates the separation of the solids from the liquid. In many processes the flocculated solids settle to form a bed by sedimentation. In other processes separation can be facilitated by mechanical dewatering, for instance in pressure filtration, centrifugation, by belt thickeners and belt presses.
The types of flocculant added to the suspension will often depend upon the substrate. Generally suspensions tend to be flocculated by high molecular weight polymers. Examples of this are described in WO-A-9314852 and U.S. Pat. No. 3,975,496 regarding the flocculation of mineral suspensions such as red mud. Other disclosures of high molecular weight polymeric flocculants include U.S. Pat. No. 6,447,687, WO-A-0216495 and WO-A-02083258 dealing with the flocculation of sewage sludge. It is known to sometimes add other chemical additives to condition the suspension. For instance suspensions may be first coagulated by a high charged density polymeric coagulant such as polyDADMAC or inorganic coagulants including ferric chloride.
Other additives are also use in conditioning of suspensions. For example peroxides are sometimes added to suspensions such as sewage sludges or other suspensions containing organic material in order to remove reducing agents in order to reduced odours, gas formation or prevent putrefaction. In general the peroxides or oxidising agents tend to be added in order to remove harmful or unwanted substances or other materials contained in the suspension. Generally the amount of peroxides added is only sufficient to remove the unwanted substances and materials and generally peroxides or other oxidising agents are included in relatively small amounts.
Examples of adding peroxides to sewage sludge are described in JP56150481. Peroxides or oxidising agents may also be added to other suspensions for similar reasons including treating dredged material to remove contaminants as described in US 2003 121863 and JP 10109100. JP 11156397 describes a process for flocculating mud using non-ionic and anionic polymers in which the mud has been pretreated with an oxidising agent.
U.S. 6733674 describes a method of dewatering sludge by adding an effective amount of one or more cellulolytic enzymes and one or more oxidants and one or more flocculants to form a mixture in water which is coagulated and flocculated followed by separation of solids from the water. The examples seem to indicate a significant time elapsed between oxidant addition and flocculation. The enzymes appeared to be present in order to degrade material contained in the sludge.
Suspensions are frequently concentrated in a gravity thickener vessel. A continual flow of the suspension is typically fed into the thickener and treated with a flocculant. The flocculated solids thus formed settle to form a bed of solid underflow and supernatant aqueous liquid flows upwards and is usually removed from the thickener vessel through a perimeter trough at the water surface. Normally the thickener vessel has a conical base such that the underflow can easily be removed from the centre of the base. In addition a rotating rake assists the removal of the underflow solids. A typical process for concentrating suspensions in a gravity thickener is described in U.S. Pat. No. 4,226,714. Various suspensions can be concentrated in gravity thickeners, including suspensions of organic solids such as wastewater, sewage and sewage sludges. If it also commonplace to thicken or dewater mineral suspensions using gravity thickeners.
In a typical mineral processing operation, waste solids are separated from solids that contains mineral values in an aqueous process. The aqueous suspension of waste solids often contain clays and other minerals, and are usually referred to as tailings. These solids are often concentrated by a flocculation process in a thickener and settle to form a bed. Generally it is desirable to remove as much water from the solids or bed in order to give a higher density underflow and to recover a maximum of the process water. It is usual to pump the underflow to a surface holding area, often referred to as a tailings pit or dam, or alternatively the underflow may be mechanically dewatered further by, for example, vacuum filtration, pressure filtration or centrifugation.
U.S. Pat. No. 5,685,900 describes a selective flocculation process for beneficiating a low brightness fine particle size kaolin in order to reduce a higher brightness kaolin clay. The process involves a classification step to recover the kaolin fraction wherein the particles are at least 90% by weight below 0.5 μm. The recovered fraction is then subjected to a bleaching step to partially bleach organic discolorants. The resulting slurry is selectively flocculated using a high molecular weight anionic polyacrylamide or acrylate acrylamide copolymer. This flocculation step forms a supernatant phase which is highly concentrated with contaminant titania and a flocced clay phase which is devoid of titania that contains the discolorants. The flocs are then treated with gaseous ozone in order to oxidising the remaining discolouring organics and also destroy the flocculant polymer in order to restore the kaolin to a dispersed state. This is said to be achieved by passing the flocculated solids through an ozonation step, preferably using a high shear pump.
Similar disclosures are made in WO 2004 071 989 and US 2006 0131243.
WO 2005 021129 discloses controlling the condition of a suspension of solid particles within a liquid including applying 1 or more stimuli to the suspension. In this disclosure conditioning is preferably reversible and involves flocculation and/or coagulation in which inter particle forces may be attractive or repulsive between the solid particles within the liquid. The stimulus may be one or more chemical additives and may for instance be a stimulus sensitive polyelectrolyte which can be absorbed on the surface of the suspended particles in sufficient quantity to create steric or electrostatic repulsion between the particles. In one instance a polyelectrolyte may be substantially insoluble at pH values where it is substantially uncharged thereby to effect flocculation of the suspension. Polyelectrolytes that are responsive to a temperature stimulus are also described. Reference is also given to a method of controlling the consolidation of a bed of solid particles within a liquid by applying one or more stimuli to the bed. Each or the stimulus effects reversibly operable conditioning between an initial state prevailing prior to said applying one or more stimuli and a conditioning state resultant from said one or more stimuli. The processes described bring about improvements in certain solids liquids separation activities.
JP 11-46541 describes a temperature sensitive hydrophilic polymer added to a suspension of particles below a transition temperature whereupon flocs are formed by absorbing and cross-linking particles as a conventional flocculant. The mixture is heated to above the transition temperature and the absorbed polymer becomes hydrophobic and the suspended particles are rendered hydrophobic and form flocs by hydrophobic interaction. Appropriate external pressure is applied at this time and the particles are readily realigned and water between the particles is expelled by the hydrophobicity of the particles. JP 2001 232104 describes a process similar to JP 11-46541 but using improved temperature sensitive flocculants that are ionic temperature sensitive polymer as opposed to non-ionic polymers which absorb onto suspended particles and when the polymer becomes hydrophobic at temperatures about the transition point there are strong hydrate layers around the ionic groups but hydrated layer adhesion between the polymers is prevented by hydrophobic interaction.
Bertini, V. et. al. Particulate Science and Technology (1991), 9(3-4), 191-9 describes the use of multifunctional polymers for the pH controlled flocculation of titanium minerals. The polymers are radical vinyl copolymers containing catechol functions and acrylic acid units. The polymers can change their effect from flocculating to dispersing or inert and vice versa by changing pH.
The pH or temperature sensitive flocculants in principle provide control over the flocculation state of a suspension. However, the choice of flocculant would need to be appropriate for the particular suspension or bed that is to be flocculated and at the same time be responsive to a particular stimulus to bring about the reversibly operable conditioning. In some cases it may be difficult to find the right choice of flocculant.
Frequently some water will be trapped in the flocculated solids and this water is often difficult to release and therefore held in the bed. Whilst pH and temperature responsive flocculants may assist with this problem it is often difficult to achieve satisfactory flocculation across a wide range of substrates.
In processes involving gravity thickeners it is desirable to operate such that the bed has the highest possible solids capable of being removed from the thickener as an underflow. Normally the limiting factor is the ability of the rake in the thickener to move the sedimented solids. It would therefore be desirable to provide a process which increases the rate of separation of the solids from the suspension and removal of the underflow.